Rationale for a new anchor design
Mooring accounts for around 15-20% of total floating wind costs. We have optimized an anchoring system for floating offshore wind by combining the versatility and low-cost of concrete materials with the efficiency of a suction anchor. Concrete can be cast in many forms and can leverage the existing local supply chain. Moving away from heavy steel-use for offshore wind structures is a trend that other experienced players in this field, e.g., Equinor and Ideol, have already embraced. Additionally we envisioned adding flotation capabilities to the anchor to allow for wet-towing. With this new design, we propose to cut costs by 80% in the anchoring systems CaPEx while maintaining all the advantages of a suction-assisted installation anchor (omni-directional loading capacity, installation accuracy, and environmentally friendly instillation and removal).
Buoyancy Chamber
The suction anchor is made of two chambers. The main suction chamber sits at the bottom. At the top of the anchor, a special buoyancy chamber (FWTC, patent pending) is realized via a pair of dome-shape walls. The domes and the hatches are designed to keep the domes at rest or under compression at all times during installation, operation, and removal. This structural arrangement allows for the minimization of the necessary reinforcement and for the creation of additional buoyancy. With an air bag creating a water-tight seal at the bottom, and with the buoyancy chamber evacuated, the anchor can be wet-towed to the installation site with inexpensive vessels.
Spiral Post-Tensioning
This new, patent-pending, three-dimensional post-tensioning arrangement allows us to mitigate longitudinal, radial, and circumferential tensile stresses that would otherwise arise in the suction skirt under certain loading conditions. This system allows for an efficient use of concrete and reduced use of steel reinforcement in the anchor, making it very economically attractive.
Padeye Solutions
At the padeye, loads from the mooring line fairlead give rise to stress concentration and localized shear and bending in the concrete walls that must be overcome with an ad-hoc structural design. FWTC conceived three patent-pending solutions: an external steel fascia with load-transferring pins fastened to a recess in the suction skirt; a tendon assisted load distribution strategy; an integrated plate that uses existing tendons.
Next Steps
Ultimate horizontal capacity vs. outer anchor diameter for different clay undrained shear strengths.
We designed the anchor for both a 10-MW and a 15-MW OWT system at sites offshore Scotland. We have also carried out a sensitivity analysis showing a wide range of soil and water conditions for this new anchor applicability. From a design standpoint, the anchor is structurally efficient and reliable; from an economic standpoint, the new anchor shows a great cost-cutting advantage for developers of floating offshore wind.
The next steps in the anchor design will require structural testing, sea-trials, and 3rd-party certification.
If interested in this technology, please contact us.